The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency. For example, SMPSs are widely used in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as a power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage.
An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
In most SMPS topologies, the voltage of the output signal, Vout is directly proportional to the voltage of the input signal, Vin:Vout∝DVin  Equation 1
In Equation 1 above, D is the duty cycle of the switching.
To minimise the difference between the actual output voltage and the desired output voltage, the duty cycle is usually controlled in dependence upon a feedback signal, wherein the feedback signal is an error signal between a measured output voltage and a desired output voltage. The error signal is fed back to a feedback compensator that controls the duty cycle so that the measured output voltage is adjusted to the desired output voltage.
It is preferable for the output signal of the SMPS to remain at its desired voltage level under all conditions. However, it is difficult to maintain the desired output voltage level due to transients on the input signal.
A transient is a change in the input signal to the SMPS. Transients on the input signal can cause the output voltage level to change almost immediately.
In known SMPS designs, only the inertia in an output filter of the SMPS will decrease this effect. In addition, the error signal fed back to the feedback compensator is often too slow in changing the duty cycle and so a large transient is introduced on the output voltage.
A known solution to the problems caused by input transients is to cascade a feed forward compensator 102 with a feedback compensator 101 as shown in FIG. 1.
In the cascade, or series, arrangement shown in FIG. 1, the feedback compensator 101 calculates a duty cycle for an SMPS (not shown in FIG. 1). The feed forward compensator 102, which is separate from the feedback compensator, calculates and applies feed forward compensation to adjust the duty cycle that has already been calculated by the feedback unit 101.
Known feed forward systems based on the arrangement of FIG. 1 are disclosed in:    Calderone, L. Pinola, V. Varoli, “Optimal feed-forward compensation for PWM DC/DC converters with “linear” and “quadratic” conversion ratio, IEEE trans, Power Electron., vol. 7, No. 2, pp 349-355, April 1992.    B. Arbetter and D. Marksimovic, “Feedforward Pulse Width Modulators for Switching Power Converters,” IEEE trans, Power Electron., vol. 12, no. 2, pp 361-368, March 1997.    M. K. Kazimierczuk, A. J. Edstron, “Open-loop peak voltage feedforward control of PWM Buck converter” IEEE trans. Circuits and Systems I, vol. 47, No. 5, pp. 740-746, May 2000.    J.-P. Sjoroos, T. Suntio, J. Kyyra, K. Kostov, “Dynamic performance of buck converter with input voltage feedforward control,” European Conference on Power Electronics and Applications, 2005.
An SMPS 100 controlled by a digital control unit 200 is shown in FIG. 2.
The voltages of the input and output signals of the SMPS 100 are sampled and converted to digital samples by analogue-to-digital converters (ADCs) 202 and 203.
Logic units 204 and 205 are used for transforming the samples into a form suitable for processing by the digital control unit and for noise filtering.
The output voltage samples from logic unit 205 are fed to the feedback compensator 206, which applies a control law such as a proportional-integral-difference (PID), also referred to as proportional-integral-differential or proportional-integral-derivative, control law.
It will be appreciated that the PID control law is just one example of a suitable control law for determining the duty cycle of a SMPS. Many alternative control laws are also possible, such as PI, PD, P, I and FIR for example.
Referring again to FIG. 2, the output from the feedback compensator 206 is adjusted by the feed forward compensator 207, in dependence upon the input voltage samples from the logic unit 204, to produce a compensated duty cycle control signal.
The feed forward compensation reduces the effects that transients on the input voltage have on the output voltage of the SMPS 100.
The compensated duty cycle control signal D is output from the digital control unit 200 and is fed to a digital pulse width modulator 208. The digital pulse width modulator 208 translates the duty cycle control signal from a digital format to a pulse width modulated (PWM) duty cycle signal. The PWM signal is then output to control the switching elements of the SMPS 100.
SMPS control units that use a feedback compensator and a feed forward compensator, such as those described above, suffer from a number of problems.
For example, the calculation of the compensated duty cycle control signal by the control unit results in long computation times and increased power consumption.
Moreover, a complex and time-consuming division operation with an additional multiplication has to be performed every switch period even when the input voltage is stable.
An additional problem with known digital feed forward compensators is that, when the voltage of the input signal is located near a quantization level of the ADC for measuring the input signal, noise can cause the quantized version of the input signal to change. The feed forward compensation will then introduce transients on the output signal even when the input signal is nearly constant.
A further problem is experienced when the input signal is changing slowly and feedback is compensating for the changes. When the voltage of the input signal then changes from one quantization level to the next, the feed forward compensation will add additional compensation which introduces transients on the output signal.
Yet a further problem with known SMPS control units is that, at light load, energy can be transferred back and forth in isolated DC/DC converters and this makes the input voltage rise. This also triggers the feed forward compensation and introduces output voltage noise.
Moreover, the inherent delay that results from taking a measurement of the input voltage Vin to performing the corrective action on the duty cycle will, in itself, cause a transient, which must be compensated for.